"It's all rather sad, actually...we are not going to engage in it, we are better than that.

"We are going to fight these elections on policy and principle."

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Speaking on Sky News' Murnaghan, Mr Clarke said: "They of course have not been able to vet their candidates. Fringe right parties do tend to collect a number of waifs and strays...

"Some of them are saying quite different things now they are in politics then their actual views."

Asked whether he agreed with David Cameron's 2006 assessment that the party was packed with "fruitcakes and closet racists", Mr Clarke replied: "I have met people who satisfy both those descriptions in Ukip.

"Indeed, some of the people who have assured me they are going to vote Ukip I would put in that category. I rather suspect they have never voted for me."

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Mr Clarke said he was sure that "most of the Ukip people are perfectly nice when they are having a drink".

But he insisted: "The trouble with Ukip really is it is just a protest party.

"It is against the political class, it is against foreigners, it is against immigrants. But it does not have any very positive policies. They do not know what they are for."

He went on: "The temptation to ordinary voters to Ukip is these are very difficult times, the political classes are regarded as having got us into a mess.

"It is very tempting to vote for a collection of clowns or indignant, angry people, who promise that somehow they will allow us to take your revenge on people who caused it."

Labour also waded into the row amid speculation that it could also lose votes to Ukip on Thursday, with frontbencher Hilary Benn telling Sky News some of the party's candidates appeared "pretty unappealing".

He said: "I think as Ukip have certainly risen in the polls they are being subjected to a lot more scrutiny, and quite right too."

Business Secretary Vince Cable added: "I think it's very important that Ukip are subject to the kind of scrutiny that we are subjected to and other major parties.

"It's absolutely right, I have never believed that it's sensible to deal with Ukip by insulting them or ignoring them.

"They are a force at the moment and they should be subject to scrutiny and subject to debate, let's try and find out what their policies are, it isn't terribly clear."

Ukip MEP Paul Nuttall claimed Labour have councillors who were former members of the far-right British National Party (BNP) and said his party would not respond to the Tories' "dirty tricks".

"If the Conservatives have got people in central office doing these kinds of dirty tricks, going through the private lives of our candidates, going back to years on social media, it's sad, it's dirty, we're not going to respond," he told BBC One's Sunday Politics.

Asked whether it was right to highlight any Ukip candidates with "neo-fascist" sympathies, he said: "We are the only political party in Britain which has it written into its constitution that if you have been a member of a far-right organisation you can never be a member of Ukip.

"The difference between ourselves and the Labour Party is this - when we find out people are members of the BNP, or have been members of the BNP, we kick them out.

"There are two Labour Party councillors out there who we know are ex-BNP members, who Labour have had elected.

"One of them was an ex-BNP councillor, so we are not going to take any lessons from anybody to fight the far-right."